A Unifying Approach to Subtidal Salt Intrusion Modeling in Tidal Estuaries

The salinity structure in estuaries is classically described in terms of the salinity structure as well mixed, partially mixed, or salt wedge. The existing knowledge about the processes that result in such salinity structures comes from highly idealized models that are restricted to either well-mixed and partially mixed cases or subtidal salt wedge estuaries. Hence, there is still little knowledge about the processes driving transitions between these different salinity structures and the estuarine parameters at which such a transition is found. As an important step toward a unified description of the dominant processes driving well-mixed, partially mixed, and salt wedge estuaries, a subtidal width-averaged model applicable to all these salinity structures is developed and systematically analyzed. Using our model, we identify four salinity regimes, resulting from different balances of dominant processes. It is shown that each regime is uniquely determined by two dimensionless parameters: an estuarine Froude and Rayleigh number, representing freshwater discharge and tidal mixing, respectively, resulting in a classification of the regimes in terms of these two parameters. Furthermore, analytical expressions to approximate the salt intrusion length in each regime are developed. These expressions are used to illustrate that the salt intrusion length in different regimes responds in a highly different manner to changes in depth and freshwater discharge. As one of the key results, we show that there are only very weak relations between the process-based regime of an estuary and the salt intrusion length and top–bottom stratification. This implies that the salinity structure of an estuary cannot be uniquely matched to a regime.

Transient Response of Atlantic Heat and Freshwater Transports in Future Climate Scenarios

<p>Ocean heat and freshwater transports play an important role in today’s climate system.  The Atlantic meridional heat transport transports 1.2 PW of heat northward leading to the warm climate we experience in Europe today, while the freshwater transport due to the Atlantic Meridional Overturning Circulation (AMOC) is often used as an indicator for the stability of the AMOC.  Future climate projections show that the AMOC is expected to weaken over the next several decades.  These changes to the AMOC as well as other circulations changes will not only impact the heat and freshwater transports in the Atlantic but also the temperature and salinity structure.  Using both CMIP5 and CMIP6 data this study untangles the impacts of velocity changes versus temperature/ salinity in future climate projections on Atlantic heat and freshwater transports.  Initial results show that changes in velocity dominate heat transport changes while the changes in salinity structure play a large role in freshwater transports with the impact of velocity changes being latitude and model dependent.</p>

BARRIER ISLAND GROUNDWATER

Storms can have long-term impacts on the groundwater flows and subsurface salinity structure in coastal aquifers. Previous studies have shown that tides, wave driven infiltration, and storm surge elevate the groundwater level within the beach (Nielsen 1999, Cartwright 2004). The resulting bulge of high groundwater propagates inland, and may cause flooding up to several days after a storm has passed (Gallien 2016). In addition, waves, tides, and storm surge force saltwater to infiltrate into the aquifer above the fresher terrestrial groundwater, and storm-driven pulses of salinity may persist for months (Robinson et al. 2014). Here, observations of groundwater heads and salinities collected continuously for three years are used to examine the effects of ocean storms, wind-driven fluctuations in sound water levels, and morphological changes on a barrier island aquifer.

A Model Study of the Discharges Effects of Kaidu River on the Salinity Structure of Bosten Lake

The salinization of Bosten Lake, which is the largest lake in the arid or semi-arid region of Xinjiang, has increased. To study the effects of the inflow change of Kaidu River, the main recharge, on the salinity structure of Bosten Lake, the Estuarine, Coastal, and Ocean Modeling System with Sediments (ECOMSED), a basic three-dimensional numerical model, was used. The model is forced by realistic atmospheric forcing and river inflows, and verified by observational data. The model simulations can map the lake water movement processes and offer an understanding of the likely role of river runoff on the Bosten Lake salinity structure. The water mainly flows eastward at the surface and westward at the bottom. The river runoff of Kaidu River significantly affects the salinity structure of the southwestern part of the lake. The Kaidu River discharge mostly flowed northeastward along the west bank of the lake, so with decreasing Kaidu River discharge, the salinity of the region from the inlet of the river to its right (looking in the direction of the flow) subsequently increased. This study helps to the mastering of the dynamic change of salinity and provides some quantity information for controlling the salinization of Bosten Lake.